I have to say that I am a little less positive as dd gets older. Like you we don't speak Welsh at home (I understand reasonably well & speak a little, DH doesn't speak Welsh at all). I think that here things are confused by the fact that dd is quite possibly somewhat dyslexic and/or dyspraxic (she is currently being assessed by the Ed Psych), so things are probably different for other children.
DD was fine in KS1 (now Foundation stage) - as you point out the 'academic' side of things moves more slowly than in England, but IMO this is much more in line with the rest of Europe; I think England is unusual in pushing formal learning very early. She was also fine learning to read, learnt fine in Welsh and transferred those skills to reading at home in English. Unsurprisingly her English reading came on very much faster than her Welsh reading (we have a lot of books at home, read together a lot, and of course her English vocab is much larger). She found learning to write harder, but with some additional support progressed at a reasonable level (a little but not a lot below average at the end of yr 2).
However, it seems to me that at dd's school at least, they don't then go through a full programme of phonics in English once they move into year 3. The children essentially seem to use their Welsh reading skills + the fact that most of them are first language English to move into English reading. Many of course like dd are already reading more in English than in Welsh. As a result, it is very hard for anyone who is not a 'natural' speller to be able to write coherently in English - they simply don't know the phonics so can't build meaningful words. There seems to be a bit of an assumption that if they read plenty, they will simply learn to spell of their own accord.
On the Welsh side, as dd has got older and the work has become more complex, the fact that she isn't using the language outside of school has also become much more of a problem. So, recently, she was working on a piece of homework which involved researching a topic. There were no books in the library on that topic in Welsh, and no resources that we could find on the internet between us in Welsh either (plenty in English). She didn't have the vocabulary to express what she wanted to say in Welsh - and so the work involved an awful lot of time simply looking up words in the dictionary. TBH, even if she spoke Welsh more outside the home, its unlikely that she would be coming across vocabulary relating to, for example, marine biology, in her everyday conversations. Whereas of course if she was habitually watching Welsh language tv, we were speaking Welsh at home etc, then she would have those words readily to hand.